Assembly of Bak homodimers into higher order homooligomers in the mitochondrial apoptotic pore

Abstract

In mitochondrial apoptosis, Bak is activated by death signals to form pores of unknown structure on the mitochondrial outer membrane via homooligomerization. Cytochrome c and other apoptotic factors are released from the intermembrane space through these pores, initiating downstream apoptosis events. Using chemical crosslinking and double electron electron resonance (DEER)-derived distance measurements between specific structural elements in Bak, here we clarify how the Bak pore is assembled. We propose that previously described BH3-in-groove homodimers (BGH) are juxtaposed via the ‘α3/α5’ interface, in which the C-termini of helices α3 and α5 are in close proximity between two neighboring Bak homodimers. This interface is observed concomitantly with the well-known ‘α6:α6’ interface. We also mapped the contacts between Bak homodimers and the lipid bilayer based on EPR spectroscopy topology studies. Our results suggest a model for the lipidic Bak pore, whereby the mitochondrial targeting C-terminal helix does not change topology to accommodate the lining of the pore lumen by BGH.

Bax, Bak and beyond — mitochondrial performance in apoptosis

“…However, the structural mechanism that mediates homodimers association into higher order oligomers remains to be defined. Different studies have identified several sites at helices α1, α3, α5, α6, and α9 where Bax and Bak oligomerize . Recently, cysteine labeling and linkage analysis of full‐length Bak in mitochondria suggested that Bak assembly into higher order oligomers proceeded via dimers association in a disordered and lipid‐mediated fashion, with no dominant dimer–dimer interface that mediates higher order oligomers formation .…”

“…According to this model, BH3‐in‐groove homodimers are juxtaposed via the ‘α3/α5 interface’, which locates on the curved surface of the pore, although its exact location remains unknown. Helices α6–α8 tether the BH3‐in‐groove region to the α9 helices, which are located at the flat region of the membrane around the lipidic pore . Interestingly, one feature that arises at all length scales of active Bax and Bak organization in the membrane is flexibility: in its structural arrangement beyond helix 6, in the poorly defined mixture of multimeric species based on dimer units and in the size and shape distribution in the macromolecular assemblies in the membrane.…”

Topology of active, membrane-embedded Bax in the context of a toroidal pore

“…Several structural and topology models of Bax and Bak have been suggested [30,34,39,[47][48][49][50]. Most models present monomers or dimers in the membrane, but fail to explain how the pore is formed.…”

“…Most models present monomers or dimers in the membrane, but fail to explain how the pore is formed. The latter information is addressed only by two available models [30,50]. Why is it difficult to reconcile the available data into a coherent structural model of active Bax (Bak) in the context of a MOM pore?…”

Parkin promotes proteasomal degradation of misregulated BAX

“…Noteworthy, the mitochondrial BAX pool prior to apoptotic stress determines the cellular predisposition to apoptosis (Reichenbach et al, 2017;Todt et al, 2013). Therefore, the large impact of the BAX α5-α6 tether on mitochondrial BAX association is consistent with BAX activation models suggesting separation between helices α5 and α6 (Bleicken et al, 2014;Czabotar et al, 2013;Mandal et al, 2016). However, BAX 5-6 commits human cells to apoptosis and even increases apoptotic activity compared with the wild-type protein.…”